1. Field of the Invention
The present invention relates to a technique controlling a drive capability of a bus such as a data bus or an address bus, and more particularly to a bus control device capable of altering a drive capability of a bus according to a region to which an access is made by a CPU (Central Processing Unit) or the like, an operating condition for the bus or the like.
2. Description of the Background Art
In recent years, a system on which a CPU is mounted has been progressed in its high functionality and multi-functionality and along this progress, various semiconductor devices have been connected to a bus in the system. Since kinds and the number of semiconductor devices connected to a bus are different according to a system and, in addition, a load capacity of a bus varies according to a system, a drive capability of an output buffer for a semiconductor device is demanded in various ways.
In such circumstances, development has been heretofore made on various bus control devices each capable of altering a drive capability of an output buffer according to a load capacity of an external bus and one example thereof includes an output buffer device with a drive capability switch function that is disclosed in Japanese Patent Laying-Open No. 10-312230. In the output buffer device with a drive capability switch function, switching is made between drive capabilities of an output buffer on the basis of a response time and a set time when an internal signal is output to an external device.
With the output buffer device with a drive capability switch function provided, unnecessary power consumption and generation of noise can be suppressed by altering an drive capability of an output buffer according to a change in a load capacity of an external bus of a semiconductor device due to changes in kind and number of devices connected to outside even if the change generates.
In the above described output buffer device with a drive capability switch function, in a case where a load capacity of a external bus of a semiconductor device takes a specific value, a drive capability of an output buffer is determined so as to satisfy a drive capability of a device with characteristics of the minimal required values for a delay time, and rise and fall times of a bus signal, of semiconductor devices connected to an external bus.
Of semiconductor devices connected to the same external bus, however, such a drive capability is in excess of requirement to a device with characteristics of larger required values for a delay time, and rise and fall times of a bus signal and the bus is driven to an excessive level when an access is made to the device, having resulted in a problem causing unnecessary power consumption and generation of noise.
Furthermore, in a case where a frequency of an operating clock for a CPU or the like is lowered in order to operate an external bus in a low power consumption mode as well, a bus is driven to an excessive level, also having led to a problem causing unnecessary power consumption and generation of noise.